By default, requests accesses user's .netrc . Disable this using the only
available (though undocumented) way: by having requests distrust the
environment. An unfortunate side-effect is that it will now also ignore
proxy settings (which do come from the environment variables, which would
be fine to access for me, unlike the .netrc).

Thus stabilising the value of these headers.

Surcharge the bot User-Agent contact url/email.

Only update the DB if user explicitely requested for this feature.
This allow to reduce the DB size increase.

This header is dropped by some CDN when getting a cache hit

This is the default Chrome behaviour

Allow empty ptr

Python seems confused with the local dns.py conflict

It's kind of requests catch-all excpetion, so print information in this case
in order to debug it better.

This allows to have better atomicity than shell redirection

TODO:
* support more TLDs

See merge request nexedi/surykatka!12

Pack dns, http, network, ssl tables.

Vacuum the DB.

* reduce information density
* skip url not to check
* warn 2 weeks before ssl certificate expiration
* skip ssl/server warning if not a critical url